A polymer electrolyte fuel cell is, for example, one prepared by sandwiching a membrane/electrode assembly between two separators to form a cell and stacking a plurality of such cells. The membrane/electrode assembly comprises an anode and a cathode each having a catalyst layer, and a polymer electrolyte membrane disposed between the anode and the cathode, to face the respective catalyst layers. The catalyst layers and the polymer electrolyte membrane contain a polymer having sulfonic acid groups, as an ion exchange resin.
A reduction reaction of oxygen at the cathode of the polymer electrolyte fuel cell proceeds via hydrogen peroxide, whereby hydrogen peroxide or peroxide radicals will be formed at the cathode. Further, to the anode, oxygen molecules from the cathode are likely to be permeated through the polymer electrolyte membrane, whereby also at the anode, hydrogen peroxide or peroxide radicals may be formed. Hydrogen peroxide or peroxide radicals formed at the cathode and the anode are known to degrade the polymer having sulfonic acid groups contained in the polymer electrolyte membrane and the catalyst layer.
The following one has been proposed as a liquid composition capable of forming a catalyst layer having resistance to hydrogen peroxide or peroxide radicals.
A liquid composition which comprises a liquid medium, a fluoropolymer comprising structural units derived from tetrafluoroethylene (hereinafter referred to as TFE) and structural units derived from a perfluorovinyl compound having a sulfonic acid group, and trivalent or tetravalent cerium ions or divalent or trivalent manganese ions (Patent Document 1).
The catalyst layer formed by using a coating liquid for forming a catalyst layer prepared by mixing such a liquid composition and a catalyst, has resistance to hydrogen peroxide or peroxide radicals. Recently, the membrane/electrode assembly is required to have a higher output voltage and to be capable of maintaining the higher output voltage for a long period of time, and even in the catalyst layer, further improvement is required.